On the other hand, youngsters who were medicated for ADHD had a raised risk of being underweight, the same researchers found.

"In light of these findings, children and adolescents with ADD/ADHD should be monitored for overweight and underweight/weight loss. By monitoring weight status of these youth, clinicians will be better prepared to prevent the development of childhood obesity and the negative physical health and psychosocial consequences," the researchers concluded.

The report is published in the July issue of Pediatrics.

In the study, researchers Molly E. Waring and Kate L. Lapane, from the department of community health at Brown Medical School in Providence, R.I., collected data on almost 63,000 children and adolescents, aged 5 to 17. The data came from the 2003-2004 U.S. National Survey of Children's Health.

The researchers found that children with ADHD who were not taking medication for the condition had a 1.5 times higher risk of being overweight, compared with children, who did not have ADHD. Conversely, children with ADHD who were on medication for the condition had a 1.6 times greater risk of being underweight, the study found.

But some experts don't find the ADHD-weight connection all that convincing.

Dr. David L. Katz, director of the Prevention Research Center at Yale University School of Medicine, notes that because childhood obesity and ADHD are both widespread, it is to be expected that some ADHD children will be obese.

"Both ADHD and obesity are highly prevalent among children and adolescent in the U.S. One would expect considerable overlap between the two conditions, even if they had little to do with one another," Katz said.

As for Ritalin and other medications used for ADHD, their association with weight is well-established, Katz said. "The answer here is to identify root causes of ADHD, so fewer children wind up needing medical treatment in the first place," he said.

Dr. David W. Goodman, an assistant professor in the department of psychiatry and behavioral sciences at Johns Hopkins University School of Medicine, Baltimore, agreed that, "the study confirms what has been known before."

Goodman contends that there is an association between obesity and ADHD, but whether it's one of cause-and-effect is unclear. "You don't have any causal link, but you can say it's a strong statistical correlation," he said.

That about one in five ADHD children are overweight is interesting but not necessarily clinically relevant, Goodman added. "We are talking about a 1.5 [times] increased risk. That's eyebrow-raising but not heart-stopping," he said.

However, Goodman believes that parents of children with ADHD should be concerned about their diet and understand that these youngsters are at higher risk of becoming obese.

"Pediatricians of newly diagnosed children with ADHD should advise parents of the risk factors for weight gain and obesity," Goodman said. "This is not simply an educational disorder, this is a disorder that affects a broad range of domains in one's life."